The invention is in the field of high resolution, high contrast image storage and display devices using lead lanthanum zirconate titanate (PLZT) ceramics.
Current image storage and display devices are limited in their applications by the intensity of the light source which must be employed. Among the image storage devices employing PLZT, ceramics, a particularly advantageous device is described in the following publications:
C. E. Land, "Information Storage and Display Techniques Using PLZT Ceramics," Proceedings of the Society of Photooptical Institute Engineers, Vol. 83, pages 44-50 (August, 1976); PA1 C. E. Land and P. S. Peercy, "New Image Storage Mechanisms in PLZT Ceramics Using Near-Ultraviolet Light," IEEE-SID Biennial Display Conference Record, pages 71-75 (October, 1976); PA1 C. E. Land and P. S. Peercy, "Photoferroelectric Image Storage in PLZT Ceramics," Information Display Journal, Vol. 13, pages 20-26 (1977); PA1 C. E. Land, "Optical Information Storage and Spatial Light Modulation in PLZT Ceramics," Optical Engineering, Vol. 17, pages 317-326 (1978); PA1 C. E. Land and P. S. Peercy, "Photoferroelectric Image Storage and Contrast Modification in PLZT Ceramics," 1978, SID International Symposium Digest of Technical Papers 9, pages 14-15, April, 1978; and PA1 C. E. Land, "Photoferroelectric Image Storage in Anti-Ferroelectric Phase PLZT Ceramics," 1978, IEEE-SID Biennial Display Research Conference Record, pages 38-41 (October, 1978).
In these articles, a device for storing high resolution, high contrast, non-volatile images is disclosed using a rhombohedral-phase PLZT ceramic plate. The PLZT ceramic plate undergoes a photoferroelectric effect (PFE). Images are stored by simultaneously exposing the image on a surface of the plate using near-UV light at or near the band gap energy of the PLZT material (about 3.35 eV or 0.37 micrometers) and switching the ferroelectric polarization through a portion of the hysteresis loop generated by an electric field generator.
More particularly, to store an image using the PFE effect, the remanent polarization of the ceramic is first switched to a point near saturation remanence. The UV image is then exposed on one of the surfaces of the ceramic and the ceramic polarization is reversed by reversing the polarity of the applied electric field. The UV illumination is removed, and the image which was stored may be exhibited as a positive image or a negative image merely by increasing, decreasing, or reversing the remanent polarization. The stored image may subsequently be switched from a positive to a negative without further exposure to UV light.
Images are stored as spatial distributions of light scattering centers and as spatial modulation of surface deformation, related to specific domain orientations in the PLZT.
The PFE image storage mechanism is based on localized domain switching which results in a spatial distribution of the light scattering centers and the surface deformation strains created during switching. The physical processes leading to localized domain switching involve optical excitation of carriers, transport of the carriers under the influence of an applied electric field, and retrapping of the carriers at new sites.
To erase the total image or a selected portion thereof, the surface of the plate to be erased is illuminated with UV light of uniform intensity, and the polarization is switched to saturation remanence.
The images may be displayed by projection through a Schlieren optical system. The image may also be displayed upon conventional photographic film as a conventional photograph.
The images can also be viewed or projected by light reflected from the surface exposed to the UV storage light.
A somewhat different type of image storage can be achieved in a penferroelectric-phase PLZT plate by simultaneously exposing the image, using near-UV light, and applying a transverse electric field. Interdigital electrodes are used to apply voltage which produces the transverse electric field. By using interdigital electrodes, the voltage required to obtain halfwave retardation in the ceramic plate depends only on the electrode spacing and the quadratic electro-optic coefficient of the PLZT. In this case, images are stored via the quadratic electro-optic effect as spatial modulation of birefringence.
The energy of the light used to form the image in the currently available PLZT devices is on the order of 85,000 microjoules per square centimeter (micro J/cm.sup.2). This energy level requires light sources higher than light energy normally available in a room lighted either by sunlight, fluorescent light, or incandescent lighting.
Another currently available image storage and display device employs a bismuth silicate material, Bi.sub.12 SiO.sub.20 (BSO) used in PROM devices. Though somewhat more sensitive than the currently available PLZT devices, the BSO image is volatile, a very undesirable characteristic.
The band gap energy of the current PLZT devices is in the near-UV range. In order to make greater use of ambient light, it would be desirable to shift the photosensitivity toward the visible region of the spectrum.
It is an object of the invention, therefore, to provide a PLZT image storage and display device having enhanced photosensitivity.
Another object of the invention is to provide an image storage and display device having non-volatile image storage, and also being more sensitive than currently available volatile image storage devices.
Another object of the invention is to provide a PLZT image storage and display device that is easily prepared and having the photoferroelectric sensitivity of the PLZT material shifted toward the visible spectrum.
Still other objects, derived benefits and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein we have shown and described only the preferred embodiments of the invention, simply by way of illustration of the best modes contemplated by us of carrying out our invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various, obvious respects, without departing from the invention. Accordingly, the drawings, Tables and description are to be regarded as illustrative in nature, and not as restrictive.